The United States went to war this morning against the militant group known as the Islamic State, as F-18s dropped bombs on targets in northern Iraq. And the question to be pondered is the one that should be considered anytime we commit our forces to combat, the one Gen. David Petraeus asked at the start of the Iraq war: “Tell me how this ends.”

The assumption is that a few air strikes – or a few dozen – can turn the tide in favor of the army of Kurdistan, the semi-autonomous state within Iraq. But given how successful the insurgents have been, particularly against a Kurdish force that was supposed to be formidable, that’s hardly certain.

President Barack Obama didn’t give any clues. Clearly he was reluctant to take this step. His aides told The New York Times that his “his hand was not forced until ISIS won a series of swift and stunning victories” over “the Kurds in the north, who have been a loyal and reliable American ally.” If this mission is about saving an ally, though, it won’t be easy to stop with air strikes if the air strikes don’t do the job.

Maybe at that point, Obama will decide he’s done all he can reasonably do and walk away. Or maybe he’ll decide he has to expand the mission. Based on our recent history, the latter is more likely, because war is our default option. And where it will lead is back into a place we shouldn't be.